1. Field of the Invention
The present invention relates to the coding, transmission and decoding of digitally coded video pictures. It more particularly relates to masking of errors in the transmission of television picture sequences in which successive pictures alternately include an intrinsically coded picture (intrapicture) and pictures differentially coded as a function of the preceding and/or following pictures (interpictures).
2. Discussion of the Related Art
A transmission technique using alternate pictures corresponds to MPEG coding, described in "MPEG: a Video Compression Standard for Multimedia Applications", by Didier Le Gall, Communications of the ACM, April 1991, vol. 34, No. 4.
FIG. 1 of the present application schematically represents a picture transmission system. This system first includes an imaging device 1 followed by an encoder 2 that compresses and codes the picture. The coded and compressed picture is conveyed by a transmission device 3 (that can be a cable link, a radio link, an optical link, etc.) associated with logic circuits to a receiver that includes a decoder and decompressor 4 (complementary to the encoder 2) and provides picture sequences to be displayed on a screen 5.
The above-mentioned MPEG standard is particularly characterized in that a sequence of pictures to be transmitted is divided into picture groups. A group includes successive pictures, the first one of which is a so-called "intra" picture, and the following ones are so-called "inter" pictures. FIG. 2 illustrates such a sequence with a first group including a first intrapicture 10 followed by interpictures 11, 12, 13 . . . , and a second group including a first intrapicture 20 followed by several interpictures 21, 22 . . .
The first picture of a group is coded in intramode, that is, according to a fixed picture compression technique based on the discrete cosine transform (DCT). Thus, the picture such as picture 10 or picture 20 is intrinsically coded through partition of the picture into blocks, cosine transform of each block, quantification of the cosine transform coefficients, and entropy coding of the quantified coefficients. Such an exemplary coding mode and device for the fabrication thereof can be found in the article "Electronic Design", May 23, 1991, pp. 49-53, by Milt Leonard.
The following pictures (11, 12, 13 . . . 21, 22 . . . ) are coded as interpictures, that is, differentially as a function of the preceding and/or following pictures. Indeed, in a normal scene viewed by a TV camera, there is little difference between a picture and the immediately preceding or following pictures, except for a few translational motions. For example, the background (landscape) can move by a translational motion due to the displacement of the camera, and some elements of the picture can themselves move by a translational motion with respect to the background (such as people or a car).
FIGS. 3A and 3B show an example of such a situation. FIG. 3A shows a frame portion partitioned into blocks Bij, and FIG. 3B illustrates a reference frame portion partitioned into blocks B'ij. When comparing both pictures, it can be seen that block B13 of FIG. 3A resembles, for example, block B'12 and that it can be deduced from block B'12 by determining a motion vector V13 and data characterizing the minor differences between blocks B'12 and B13. Similarly, block B26 corresponds to a picture portion consisting of a part of each of blocks B'23, B'24, B'33 and B'34, and can be deduced therefrom by a motion vector V26 and difference data. Additionally, it will be understood that, generally, immediately adjacent blocks are very likely to be associated with substantially identical motion vectors. Coding of a picture in inter mode includes the following steps:
partitioning into blocks, PA1 estimating the motion as compared to the adjacent pictures in order to provide one or several motion vectors with respect to each of these adjacent pictures, PA1 extracting a predictor block from one or several adjacent pictures, PA1 quantifying the transform coefficients, PA1 entropically coding the predictor displacement and the quantified coefficients with respect to the block.
carrying out the cosine transform of the difference between each block and its predictor,
A description of an inter coding method and of a device for the implementation thereof can be found in the article by Milt Leonard, "Electronic Design", April 2, 1992, pp. 45-54.
The transmission device 3 receives a bit string provided by the encoder and the partition into data packets. As shown in FIG. 4, the transmission device 3 adds to each data packet P1, P2, P3 . . . identification codes and an error detection/correction code. Then, the data packets are transmitted and, upon reception, data packets are recovered, their error detection/correction code is analyzed and their content is provided to the decoder.
Upon reception of a data packet, when the transmission device 3 detects one or several transmission errors in the data packet, either it can correct these errors (this operation is automatically carried out without action of the decoder), or its error correction capacity is exceeded; in that case, the transmission device signals to the decoder that a data packet is lost. It is then no longer possible to correct the error but it can be attempted to mask the consequences thereof (it should be noted that a transmitted data packet generally corresponds to several blocks resulting from the above-mentioned partitioning into blocks).
In the case of an intra-coded picture, one solution provides for replacing the missing blocks with blocks placed at the same position in a preceding picture. However, this masking step may prove inappropriate if the picture in error corresponds to a picture that rapidly moves with respect to the preceding picture.
In the case of inter-coded pictures, since the motion vectors adjacent to the missing blocks are available, it is possible to estimate the motion vectors and the difference coefficients of the missing blocks, for example by averaging the motion vectors and difference coefficients of the adjacent blocks, and thus to replace the missing blocks by blocks from a preceding picture with estimated motion vectors and difference coefficients.
Therefore, the masking of missing blocks is not so good in the case of intra-pictures as in the case of interpictures. This is particularly impairing because errors on intrapictures are more detrimental than errors on interpictures since they have repercussions, step by step, on the next inter-coded pictures whose coding is carried out from these intrapictures.
Therefore, in the prior art, the error correction systems have been enhanced for intrapictures, but such enhancement is not limitless because it requires to increase the number of bits assigned to the detection and correction of errors in each data packet of the transmission device and to substantially increase the complexity of the hardware assigned to the processing of errors.